Having excellent mechanical properties and chemical resistance, aromatic polyesters typified by polyethylene terephthalate (PET) have been used widely as fibers and molded products such as films. Especially, being economical and excellent in terms of sanitary, a PET resin has been used widely as food containers, particularly, beverage containers. These containers are commonly referred to as “PET bottle” and their consumption quantity is immense. Further, from the viewpoint of an increase in recent concern on environmental issues and saving of resources, regarding used beverage PET bottles, recycling has been promoted so far. Its application method has drawn attention.
A film including PET is produced, in general, by melt-extruding PET out of a cap, obtaining an undrawn film by quenching the extruded film-like melt on a rotating cooling drum surface, and successively, drawing the undrawn film in longitudinal and width directions. In this case, in order to eliminate surface defects and to enhance the thickness evenness of the film, the adhesiveness between the film-like melt and the surface of the rotating cooling drum needs to be improved.
In order to solve the problem, at the time of quenching an extruded sheet-like object melt-extruded out of an extrusion cap on the surface of a rotating cooling drum, it is required to improve the adhesiveness between the sheet-like object and the drum surface. As a method for improving the adhesiveness between the sheet-like object and the drum surface, a method (an electrostatic adhesion casting method) is effective in which a wire-like electrode is installed between the extrusion cap and the rotating cooling drum, high voltage is applied, static electricity is generated on the uncured sheet-like object surface, and the sheet is quenched while the sheet is attached to the cooling body surface.
In order to efficiently carry out the electrostatic adhesion casting method, in other words, it is required to improve the electrostatic adhesiveness between the sheet-like object and the drum surface, and because of this, how high quantity of electric charge can be generated on the sheet-like surface is important. In order to increase the electric charge, it is effective to lower the specific resistance of a polyester by modifying the polyester, and good deal of effort has been made.
For example, it is disclosed that the specific resistance is lowered by adding a magnesium compound and a phosphorus compound in a manner of adjusting the atomic ratio of magnesium atoms and phosphorus atoms in a specified range at the time of producing PET (see Patent Document 1). According to the above-mentioned document, it is disclosed that foreign materials derived from a catalyst can be decreased and also the quality of a film can be improved by specifying the timing of addition of a magnesium compound, a sodium compound, and a phosphorus compound.
On the other hand, it is not originally intended to use PET reproduced from recycled PET bottles for films and therefore, the PET has a high specific resistance value and is indispensably necessary to be modified for improvement of electrostatic adhesiveness in terms of film productivity.
Furthermore, recycled PET bottles tend to be polluted and contaminated easily with foreign materials such as attached labels, printing inks used for the attached labels, and dust attached to the bottles, and in the case of production of films from starting materials derived from the recycled PET bottles, these foreign materials become defects and cause loss of quality of the films and consequently, it may probably result in inferior appearance in the case where the films are used as a wrap-around beverage PET bottle label. Consequently, in the case of such products, the meaning of saving a resource is possibly lessened, and development of products which reduce load on environments without lowering their quality has been increasing (see Patent Document 2).
Moreover, there is another problem that being an insulator, a conventional film tends to generate and accumulate static electricity. The static electricity is a cause of problems at the time of processing, for example, it causes a problem of wrapping of the film around a roll, generating shock on the human body with which the film is brought into contact, or making it difficult to handle the film to lower the workability in production step or in secondary processing step such as printing, adhesion, and the like. Further, it becomes a cause of, so-called, generation of thin lines at printing, inferior of attachment property after label formation from the film, or stains of the film surface, and it accordingly lowers the product value.